1. Field of the Invention
The present invention relates to a handover in or between wireless cellular or personal communications Code Division Multiple Access (CDMA) systems wherein a mobile station is switched or transferred from one set of forward and reverse communication links to another. In its particular aspects, the present invention relates to a handover which at least requires the mobile station to change its operating frequency, referred to herein as an xe2x80x9cinter-frequency handoverxe2x80x9d.
2. Description of the Related Art
A method for such a handover is known from A. Baier et al., xe2x80x9cMULTI-RATE DS-CDMA RADIO INTERFACE FOR THIRD-GENERATION CELLULAR SYSTEMSxe2x80x9d, Seventh IEE European Conference on Mobile and Personal Communications, 13-15 December 1993, pp. 255-260.
Handovers are employed in wireless cellular and personal communications systems to allow mobile stations to travel from the coverage area of one base station to another while maintaining a call. While handovers are usually employed to transfer an ongoing communication with the mobile station from a current to a new base station, it is also possible to hand over a communication with the mobile station from one set of forward and reverse links to another of the same base station.
In a CDMA system a handover of a mobile station between base stations can be either a soft handover, in which during a transient period the mobile station simultaneously maintains forward and reverse links with both the current and new base stations, generally at the same frequency, and receives the same data via both base stations, and a hard handover, where the mobile station switches from current links to new links generally at a new frequency and/or of a new network without such a transient period in which current and new forward and reverse links are simultaneously maintained.
Unfortunately, inter-frequency handovers are subject to dropped calls causing annoyance and inconvenience to the parties to the dropped calls.
One method of improving the reliability of an inter-frequency handover is to use a transient compressed mode in which the data transmitted to the current base station at the current frequency F1 is squeezed into the first halves of frames by doubling the instantaneous symbol rate RINFO while allowing the mobile station to tune to and establish communication at the new frequency F2 in the second halves of frames. While the compressed mode allows for a more gradual inter-frequency handover similar to a soft handover, there may be a deterioration of the bit error (BER) during the time that this mode is employed due to the doubling of the instantaneous symbol rate. Since the chip rate RCHIP of the Pseudo Noise (PN) spreading code is held constant, the doubling of the symbol rate RINFO results in a halving of the spreading factor RCHIP/RINFO in each half of the frame. To prevent the bit error rate BER from increasing due to this halving of the spreading factor, the power level of transmissions in compressed mode is increased accordingly. However, it is not always possible to sufficiently further increase the power level of transmission, e.g. when the power level of the mobile station is already at or near its maximum radiated power. Under such circumstances, the compressed mode may also cause a dropped call.
It is an object of the present invention to reduce the occurrence of dropped calls as a result of inter-frequency handover in CDMA wireless cellular and personal communication systems, as an alternative to increasing transmission power during the handover, and also where it is not possible to sufficiently increase the transmission power during the handover.
The present invention is based on the insight that to substantially obviate a high risk of a dropped call during an inter-frequency handover, the BER should in fact be significantly reduced (rather than merely maintained) by increasing the spreading factor of transmissions to and from the mobile station.
Thus, a handover method in accordance with the present invention with respect to acts occurring at, or from the point of view of, the mobile station is characterized in that in response to a handover command from the current base station identifying a new operating frequency different from the current operating frequency, a final spreading factor, and a starting spreading factor which is at least double the final spreading factor, the operating frequency of the mobile station is switched in accordance with the identified new operating frequency, the mobile station transmits during a transient period at the identified starting spreading factor, forward and reverse links from the mobile station at the new operating frequency are established by the end of the transient period, and the mobile station communicates over the established new links at the identified final spreading factor after the end of the transient period. Also, an apparatus comprising a programmed microprocessor or controller for a mobile station in accordance with the present invention is characterized in that it is programmed to steer the foregoing acts in response to an inter-frequency handover command.
Further, in accordance with the present invention, a method with respect to acts occurring at, or from the point of view of, the current base station is characterized in that the current base station transmits to the mobile station a handover command identifying a new operating frequency different from the current operating frequency, a final spreading factor, and a starting spreading factor which is at least double the final spreading factor.
The methods and apparatus of the present invention are additionally characterized in that the starting spreading factor is at least four times the final spreading factor. Another aspect of the present invention is that the transient period is of a predetermined fixed duration.
Other objects, features and advantages of the present invention will become apparent upon perusal of the following detailed description when taken in conjunction with the appended drawing, wherein: